What is Palestinian Islamic Jihad?

ON THE EVENING of October 17th a blast struck al-Ahli hospital in Gaza City. Thousands of civilians had been sheltering there, alongside the sick. Palestinian officials say that hundreds died. Hamas, the militant group that runs Gaza, and several Arab countries blamed Israel. Israel’s army, citing drone footage as evidence, said that the explosion was caused by a malfunctioning rocket fired by Palestinian Islamic Jihad (PIJ), another militant group. PIJ, which operates in the West Bank and Gaza, denied the allegation. On October 18th, on a visit to Israel, President Joe Biden also blamed “the other team”, backing up Israel’s account. What is PIJ and what threat does it pose in this conflict?